Under certain conditions galvanic cells may generate large quantities of gas. As these cells are sealed in order to prevent loss of electrolyte, such gas generation may result in the formation of high pressures within such cells. If such high pressures are not vented, cell leakage, bulging and/or rupture may occur.
In the past many approaches have been adopted for releasing the internal pressure which may build up in galvanic cells. In general, it is preferable to employ resealable venting means in order to avoid drying of the cell's electrolyte and to prevent the ingress of oxygen from the atmosphere which can cause wasteful corrosion of the anode. Prior resealable venting means have generally employed a deformable plastic member located at the interface of the seal closure member and the metal container wall of the cell.
For example, U.S. Pat. Nos. 4,020,241 and 4,063,902 disclose a galvanic cell having a resealable vent closure consisting of a resilient elastomeric sponge gasket disposed and compressed between the cover of the cell and the upper wall of the cell's container. This vent closure is designed to vent low pressure gas buildup along the cover-gasket interface and/or container-gasket interface.
U.S. Pat. No. 4,079,172 discloses sealed galvanic dry cells having at least one gas venting passage disposed at the interface of the top surface of the cover and the curled-over rim of the container. The passage is defined as being a recess in the cover disposed below the curled-over rim and/or a notch in a portion of the curled-over rim.
U.S. Pat. No. 4,074,022 discloses sealed electric cells wherein the cell casing is fitted with a vent which consists of a tongue formed in a metal portion of the casing whose inner surface is in contact with a seal. The non-detached tongue in the casing wall is turned towards the enclosed volume of the casing. In the case of internal overpressure, the seal is deformed and allows the interior of the electric cell to communicate with the exterior.
U.S. Pat. No. 3,802,923 discloses a seal closure for the open end of a cell comprising a cover including an annular depending flange which has a circumferential notch for engaging the inwardly turned peripheral edge of the cell and a resilient sealing lip which resides on the curled over top portion of the container. Although the seal closure of this patent provides satisfactory results when used in conjunction with a sealant, cells employing such rim vent seals require special equipment not generally commercially available for their assembly.
Although prior art venting means such as those described above are effective for releasing excess interior pressure from galvanic cells, a major disadvantage of these venting means is that the excess pressure is released via a metal-plastic interface. A metal-plastic interface is not as desirable as a plastic-plastic interface for several reasons. For example, electrolyte may creep along metal surfaces due to electrocapillary action preventing the metal-plastic seal from resealing properly. Moreover, in cell containers comprising a consumable anode, such as zinc, the cell container tends to be corroded during the lifetime of the cell. This could lead to scoring or pitting of the metal at the plastic-metal interface and produce a corresponding reduction in seal effectiveness. Although it has been found that the use of a greasy sealant adequately protects the metal can in certain of the prior art venting means, particularly that described in U.S. Pat. No. 3,802,923, it is nevertheless desirable to possess alternative effective resealable venting means.
It is therefore an object of this invention to provide a closure member having resealable venting means which contains a "dry" all-plastic vent path for the release of excess pressure from the interior of a galvanic cell.
It is a further object of this invention to provide a closure member for galvanic cells which contains a resealable venting means, which cells may be readily assembled utilizing conventional equipment.
It is a further object of this invention to provide a method for the manufacture of galvanic cells which possess a resealable all-plastic vent path.
The foregoing and additional objects will become more fully apparent from the following description and the accompanying drawing.